In a study of 11 patients in Shanghai, Chinese doctors found that
two samples of the virus have developed a mutation that makes
them resistant to the drug — meaning the treatment was rendered
ineffective.

In one patient, they even saw the resistance take hold after
initial infection. The virus adapted to treatment with the drug
while it was infecting the patient's body. After nine days of
treatment with the drug, the virus developed a mutation in the
neuraminidase gene, the N in H7N9. This mutation made the drug
ineffective.

The patients with the mutation were the ones who did the worst
after getting to the hospital —
they ended up on breathing machines and on machines that
oxygenated their blood outside of their body, called ECMO. One
died.

"The apparent ease with which antiviral resistance emerges in
A/H7N9 viruses is concerning; it needs to be closely monitored
and considered in future pandemic response plans," the
researchers wrote in an
article published online by The Lancet [PDF] medical journal
on Tuesday, May 28.

The virus, which seems to come from birds, has infected 133
people and killed 36, according to FluTrackers. There could me more
cases lurking out there that haven't tested positive. Cases have
been slowing in recent weeks, but the threat that it could mutate
and infect more people remains as long as it's still
circling in animal populations.